This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP02/01730 which has an International filing date of Feb. 26, 2002, which designated the United States of America.
The present invention relates to an exhaust emission control device of an internal-combustion engine, and more specifically, to a technique for identifying lowering of the performance of exhaust emission control means capable of reducing harmful substances in exhaust gas.
An exhaust emission control technique utilizing reaction on a catalyst is known as a technique that is intended to reduce harmful substances (including smoke, NOx, etc. as well as unburned substances such as HC, CO, H2, etc.) in exhaust gas.
However, this exhaust emission control technique has a problem that the unburned substances including HC and the like are discharged into the atmosphere before the catalyst in a cold state is activated.
Described in Jpn. Pat. Appln. KOKAI Publications Nos. 3-117611 and 4-183921, therefore, is a developed technique such that the exhaust pressure is raised to activate the catalyst early.
According to this technique, a closed variable-flow valve is provided on the lower-stream side of an exhaust passage, for example. By adjusting the closed variable-flow valve to change the sectional area of the exhaust passage, the exhaust resistance and exhaust density can be increased, or the exhaust flow rate can be lowered to increase the exhaust pressure and exhaust gas temperature.
If the exhaust gas pressure is increased in this manner, individual performances can be enhanced in the cases where unburned substances are fed into an exhaust system (two-stage fuel injection, etc.), EGR (exhaust gas recirculation) is carried out, and exhaust air-fuel ratio modulation (exhaust A/F modulation) is carried out. Thus, reaction of the unburned substances in the exhaust passage including a combustion chamber is accelerated to raise the exhaust gas temperature, so that the catalyst is activated early enough to promote purification of the exhaust gas drastically.
Since the closed variable-flow valve is placed in a high-temperature, high-pressure environment, it sometimes may go wrong and cease to operate normally after prolonged use.
If the valve of the closed variable-flow valve thus ceases to operate normally, the valve may not be able to be closed, so that the actual exhaust pressure fails to rise, although the exhaust pressure is expected to be increased. Otherwise, the valve may be left closed inevitably to keep the exhaust pressure high, although the exhaust pressure is not expected to be increased.
Accordingly, there is an idea of providing an exhaust pressure sensor in the exhaust passage and detecting something abnormal about exhaust control means, such as the closed variable-flow valve, in accordance with exhaust pressure information that is detected by means of the exhaust pressure sensor.
On other hand, exhaust gas that is discharged from a diesel engine that is mounted in a vehicle contains plenty of particulate matter (abbreviated PM) as well as HC, CO, etc. A diesel particulate filter (abbreviated DPF) for capturing the PM and removing it by combustion by means of an external heat source or the like is practically used as a post-exhaust processor of the diesel engine.
Thus, noises and error factors other than the exhaust pressure that are involved in the output signal of the exhaust sensor can be eliminated by obtaining the relations between a plurality of pieces of output information with different exhaust component concentrations (exhaust A/F""s) from the exhaust sensor and a plurality of target outputs with the individual exhaust component concentrations (individual exhaust A/F""s) under the reference pressure (e.g., atmospheric pressure or the like), e.g., the ratios between the differences between a plurality of exhaust outputs and the differences between a plurality of target outputs. By doing this, the exhaust pressure can be estimated more properly. Lowering of the performance of the exhaust emission control means, which can reduce the harmful substances in the exhaust gas under the given exhaust pressure condition, can be accurately identified by monitoring the relations between the outputs of the exhaust sensor and the target outputs.
The PM captured and deposited on the filter should preferably be removed by combustion to regenerate the filter before the PM capture limit is reached, and the exhaust pressure on the upper-stream side of the post-exhaust processor rises depending on the quantity of the PM deposited on the filter. In consideration of this fact, as is described in Jpn. Pat. Appln. KOKAI Publication No. 8-303290, for example, the exhaust pressure sensor is provided in the exhaust passage on the upper-stream side of the post-exhaust processor so that the PM can be removed by combustion on the assumption that the PM capture limit is approached when a given exhaust pressure is reached by the exhaust pressure that is detected by means of the exhaust pressure sensor.
In the case where the exhaust pressure sensor is used in this manner, however, the exhaust pressure sensor must be provided separately. In general, an exhaust pressure sensor has a specification such that it can resist high temperature and high pressure, it is expensive, and therefore, the parts cost is high.
The object of the present invention is to provide an exhaust emission control device of an internal-combustion engine, whereby lowering of the performance of exhaust emission control means, which can reduce harmful substances in exhaust gas, can be securely identified at low cost.
In order to achieve the above object, an exhaust emission control device according to the present invention comprises exhaust emission control means located in an exhaust passage of an internal-combustion engine and capable of reducing harmful substances in exhaust gas under a given exhaust pressure condition, an exhaust sensor for detecting the concentration of a specific exhaust component in the exhaust gas, and performance lowering identifying means for identifying lowering of the performance of the exhaust emission control means by detecting a failure to fulfill the given exhaust pressure condition in accordance with the output of the exhaust sensor.
Thus, the exhaust pressure can be estimated in accordance with information from the existing exhaust sensor without separately providing an exhaust pressure sensor, and lowering of the performance of the exhaust emission control means, which can reduce the harmful substances in the exhaust gas under the given exhaust pressure condition, can be securely identified at low cost by monitoring the output of the exhaust sensor and detecting the failure to fulfill the given exhaust pressure condition.
The present invention is based on the following knowledge, which will be described below.
In general, the diffusing speed of each exhaust component increases to raise its partial pressure if the exhaust pressure rises.
On the other hand, the following Nernst formula (1) is generally known as a theoretical formula for an O2 sensor output in the case where the exhaust sensor is an O2 sensor, for example.
xe2x80x9cNernst formulaxe2x80x9d: electromotive force (O2 sensor output)=(gas constantxc3x97operating temperature/(4xc3x97Faraday constant)xc3x971 n(atmosphere-side O2 partial pressure/exhaust-side O2 partial pressure).xe2x80x83xe2x80x83(1) 
Here the exhaust-side O2 partial pressure is given by the following formula (2):
exhaust-side O2 partial pressure=exhaust pressurexc3x97exhaust O2 concentration.xe2x80x83xe2x80x83(2) 
This Nernst formula indicates the following phenomenon. If the exhaust pressure increases, the exhaust-side O2 partial pressure increases to cause the electromotive force (O2 sensor output) to lower on the high-O2 lean-A/F side, while the H2 partial pressure and the exhaust-side O2 partial pressure increases and reduces, respectively, to cause the electromotive force (O2 sensor output) to grow on the high-H2 rich-A/F side.
This implies that the variation of the exhaust pressure can be detected by monitoring the electromotive force (O2 sensor output) and that something abnormal about the exhaust control means can be detected by comparing the output with a target output under the atmospheric pressure, for example.
In the case where the exhaust sensor is an A/F sensor, moreover, the following pump-current formulas (3) and (4) are generally known as theoretical formulas for the A/F sensor output.
xe2x80x9cPump-current formulasxe2x80x9d: lean-side pump current 
(A/F sensor output)=(4xc3x97Faraday constantxc3x97O2 
diffusion constantxc3x97sectional area of diffusion passage 
opening/(gas constantxc3x97operating temperaturexc3x97length of 
diffusion passage)xc3x97(exhaust-side O2 partial 
pressurexe2x88x92O2 partial pressure on gas detecting 
chamber side (atmosphere side)),xe2x80x83xe2x80x83(3) 
rich-side pump current (A/F sensor output)=(2xc3x97Faraday constantxc3x97sectional area of diffusion passage opening/(gas constantxc3x97operating temperaturexc3x97length of diffusion passage)xc3x97(H2 diffusion constantxc3x97exhaust-side H2 partial pressurexc3x97CO diffusion constantxc3x97exhaust-side CO partial pressure).xe2x80x83xe2x80x83(4) 
Here the pump current on the lean side and the pump current on the rich side are positive (+) and negative (xe2x88x92), respectively, or negative (xe2x88x92) and positive (+), respectively.
These pump-current formulas indicate the following phenomenon. If the exhaust pressure increases, the exhaust-side O2 partial pressure increases to cause the pump current and the A/F sensor output to increase on the high-O2 lean-A/F side, while the H2 partial pressure increases to cause the pump current and the A/F sensor output to increase in the opposite direction and reduce, respectively, on the high-H2 rich-A/F side.
This implies that the variation of the exhaust pressure can be detected by monitoring the pump current (A/F sensor output) and that something abnormal about the exhaust control means can be detected by comparing the output with a target output under the atmospheric pressure, for example.
The exhaust sensor may be an NOx sensor, and in this case, the aforesaid pump-current formulas are also applicable as theoretical formulas.
In the exhaust emission control device of the present invention, moreover, the exhaust emission control means includes an exhaust flow control valve capable of adjusting the flow area of the exhaust passage, and the performance lowering identifying means identifies something abnormal about the exhaust flow control valve.
Accordingly, the exhaust pressure rises as the exhaust flow control valve is closed, whereupon reaction of unburned substances in the exhaust passage is accelerated to increase the exhaust gas temperature. Thus, a catalyst is activated early, so that the exhaust emission control efficiency is improved. As this is done, the exhaust pressure can be estimated in accordance with information from the existing exhaust sensor, and something abnormal about the exhaust flow control valve can be securely identified at low cost by monitoring the output of the exhaust sensor.
In the exhaust emission control device of the present invention, moreover, the performance lowering identifying means identifies lowering of the performance of the exhaust emission control means by comparing the output from the exhaust sensor and a target output with the same exhaust component concentration under a reference pressure.
Thus, the exhaust pressure can be properly estimated with ease by comparing the output information from the exhaust sensor and the target output with the same exhaust component concentration (same exhaust A/F) under the preset reference pressure (e.g., atmospheric pressure or the like). Lowering of the performance of the exhaust emission control means, which can reduce the harmful substances in the exhaust gas under the given exhaust pressure condition, can always be satisfactorily identified by monitoring the respective comparative values of the output of the exhaust sensor and the target output.
In the exhaust emission control device of the present invention, moreover, the performance lowering identifying means identifies lowering of the performance of the exhaust emission control means in accordance with relations between a plurality of outputs with different exhaust component concentrations from the exhaust sensor and a plurality of target outputs with the individual exhaust component concentrations under the reference pressure.
Thus, noises and other error factors than the exhaust pressure that are involved in the output signal of the exhaust sensor can be eliminated by obtaining the relations between a plurality of pieces of output information with different exhaust component concentrations (exhaust A/F""s) from the exhaust sensor and a plurality of target outputs with the individual exhaust component concentrations (individual exhaust A/F""s) under the reference pressure (e.g., atmospheric pressure or the like), e.g., the ratios between the differences between a plurality of exhaust outputs and the differences between a plurality of target outputs. By doing this, the exhaust pressure can be estimated more properly. Lowering of the performance of the exhaust emission control means, which can reduce the harmful substances in the exhaust gas under the given exhaust pressure condition, can be accurately identified by monitoring the relations between the outputs of the exhaust sensor and the target outputs.
In the exhaust emission control device of the present invention, moreover, the exhaust sensor has a characteristic to detect two or more exhaust component concentrations and is adapted to detect the concentration of at least one specific exhaust component when the exhaust air-fuel ratio is a lean air-fuel ratio and to detect the respective concentrations of other specific exhaust components when the air-fuel ratio is a rich air-fuel ratio.
The exhaust gas contains much O2 with a lean air-fuel ratio and much H2 with a rich air-fuel ratio, for example. If one specific exhaust component (O2 component) with the lean air-fuel ratio and another specific component (H2 component) with the rich air-fuel ratio can be individually detected by means of the exhaust sensor, therefore, the exhaust pressure can always be satisfactorily estimated without regard to the air-fuel ratio, lean or rich. Thus, lowering of the performance of the exhaust emission control means, which can reduce the harmful substances in the exhaust gas under the given exhaust pressure condition, can always be identified in a wide air-fuel ratio range without regard to the air-fuel ratio by monitoring the output of the exhaust sensor.
In the exhaust emission control device of the present invention, moreover, the exhaust emission control means is a post-exhaust processor, and the performance lowering identifying means determines the necessity of regeneration of the post-exhaust processor.
If the quantity of PM deposited on a filter of the post-exhaust processor increases, the exhaust pressure on the upper-stream side of the post-exhaust processor is raised as the filter is clogged. However, the exhaust pressure can be estimated in accordance with information from the existing exhaust sensor without separately providing an exhaust pressure sensor. Thus, the necessity of regeneration of the post-exhaust processor can be securely determined at low cost by monitoring the output of the exhaust sensor.
In the exhaust emission control device of the present invention, moreover, the performance lowering identifying means sets a decision value for determining the necessity of regeneration of the post-exhaust processor in accordance with the operating conditions of the internal-combustion engine and the specific exhaust component concentration, and determines the necessity of regeneration of the post-exhaust processor by comparing the output of the exhaust sensor and the decision value using the same operating conditions and the same exhaust component concentration.
Thus, the exhaust pressure can be properly estimated with ease by comparing the output information from the exhaust sensor and the decision value using the same preset operating conditions and the same exhaust component concentration, and the necessity of regeneration of the post-exhaust processor can always be satisfactorily determined by comparing the respective comparative values of the output of the exhaust sensor and the decision value.
In the exhaust emission control device of the present invention, moreover, the exhaust sensor is an O2 sensor, the post-exhaust processor includes an occlusion-type NOx catalyst, and the performance lowering identifying means determines the necessity of regeneration of the post-exhaust processor when the exhaust air-fuel ratio is adjusted to a rich air-fuel ratio in order to release NOx from the occlusion-type NOx catalyst.
If exhaust pressure increases when the exhaust A/F is a rich air-fuel ratio, the H2 partial pressure increases to cause the O2 sensor output to increase. If the O2 sensor is used because the extent of the change is higher than that of the change of the O2 partial pressure with a lean air-fuel ratio owing to the character of the O2 sensor, the necessity of regeneration of the post-exhaust processor can be securely determined with ease by monitoring the output of the O2 sensor when the exhaust air-fuel ratio is adjusted to a rich air-fuel ratio in order to release NOx from the occlusion-type NOx catalyst.
In the exhaust emission control device of the present invention, moreover, the performance lowering identifying means sets, in accordance with the operating conditions of the internal-combustion engine, a target output under a reference exhaust pressure with the post-exhaust processor nonoperating, sets a decision value for determining the necessity of regeneration of the post-exhaust processor in accordance with the exhaust component concentration, and concludes that regeneration of the post-exhaust processor is necessary if the deviation between the output of the exhaust sensor and the target output exceeds the decision value with the same operating conditions and the same exhaust component concentration.
Thus, the exhaust pressure can be properly estimated with ease by comparing the deviation between the output information from the exhaust sensor and the target output under the preset reference exhaust pressure with the decision value with the same operating conditions and the same exhaust component concentration, and a high-exhaust-pressure situation that requires regeneration of the post-exhaust processor can be identified with ease by detecting the attainment of the decision value by the deviation.
In the exhaust emission control device of the present invention, moreover, the exhaust sensor is a linear air-fuel ratio sensor, and the performance lowering identifying means determines the necessity of regeneration of the post-exhaust processor when the exhaust air-fuel ratio is a lean air-fuel ratio during injection quantity feedback control using the linear air-fuel ratio sensor and based on the excess air factor of the internal-combustion engine.
When the exhaust A/F is a lean air-fuel ratio, compared with a rich air-fuel ratio, the higher the degree of leanness, the higher the extent of change of the linear air-fuel ratio sensor output that corresponds to the change of the exhaust pressure is. In the case where the linear air-fuel ratio sensor is used, therefore, the necessity of regeneration of the post-exhaust processor can be securely determined with ease by monitoring the output of the linear air-fuel ratio sensor when the exhaust A/F is a lean air-fuel ratio during the injection quantity feedback control based on the excess air factor of the internal-combustion engine.
In the exhaust emission control device of the present invention, moreover, the exhaust sensor is a linear air-fuel ratio sensor, and the performance lowering identifying means determines the necessity of regeneration of the post-exhaust processor when the exhaust air-fuel ratio is a lean air-fuel ratio during exhaust reflux feedback control using the linear air-fuel ratio sensor.
When the exhaust A/F is a lean air-fuel ratio, compared with a rich air-fuel ratio, the higher the degree of leanness, the higher the extent of change of the linear air-fuel ratio sensor output that corresponds to the change of the exhaust pressure is. In the case where the linear air-fuel ratio sensor is used, therefore, the necessity of regeneration of the post-exhaust processor can be securely determined with ease by monitoring the output of the linear air-fuel ratio sensor when the exhaust A/F is a lean air-fuel ratio during the exhaust reflux feedback control.
In the exhaust emission control device of the present invention, moreover, the exhaust sensor is a linear air-fuel ratio sensor, the post-exhaust processor includes an occlusion-type NOx catalyst, and the performance lowering identifying means determines the necessity of regeneration of the post-exhaust processor when the exhaust air-fuel ratio is adjusted to a rich air-fuel ratio in order to release NOx from the occlusion-type NOx catalyst.
Thus, in the case where the linear air-fuel ratio sensor is used, the necessity of regeneration of the post-exhaust processor can be securely determined with ease by monitoring the output of the linear air-fuel ratio sensor when the exhaust A/F is adjusted to a rich air-fuel ratio in order to release NOx from the occlusion-type NOx catalyst as well as when the exhaust A/F is a lean air-fuel ratio.